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Full text of "A low-cost hydraulic cylinder"

A 



A LOW-COST 
HYDRAULIC CYLINDER 




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WEST VIRGINIA UNIVERSITY 
AGRICULTURAL EXPERIMENT STATION 
BULLETIN 493T, MAY 1964 



Digitized by the Internet Archive 

in 2010 with funding from 

Lyrasis Members and Sloan Foundation 



http://www.archive.org/details/lowcosthydraulic493phil 



LOW-COST 
HYDRAULIC CYLINDER 



Ross A. Phillips 



WEST VIRGINIA UNIVERSITY 
AGRICULTURAL EXPERIMENT STATION 



THE AUTHOR 

Ross A. Phillips is Assistant Professor 
of Agricultural Engineering and Assistant 
Agricultural Engineer in the Agricultural 
Experiment Station. 



This study was part of a North- 
east Regional Project— "The Mech- 
anization of Forage Crop Harvest- 
ing, Processing, Storing and Feed- 
ing," NE-13, a cooperative study 
involving agricultural experiment 
stations in the Northeast Region 
and supported in part by regional 
funds. 



West Virginia University 

Agricultural Experiment Station 

College of Agriculture, Forestry, and Home Economics 

A. H. VanLandingham, Director 

Morgantown 



A LOW-COST 
HYDRAULIC CYLINDER 



H 



YDRAULIC cylinders are 
used extensively wherever linear 
motion with high force and low 
velocity is required. This re- 
quirement was encountered in 
designing an experimental hay 
drier where a linear motion 
through a distance of about five 
feet was needed. Desired forces 
were estimated to be around 
5,000 pounds, to be applied on 
the end of a bale of hay. 

Various mechanical devices 
were considered but all were 
either too cumbersome or inter- 
fered with the working space 
near the bale of hay. Room was 
available for a long cylinder, 
and cost estimates were obtain- 
ed for a suitable commercial cyl- 
inder. These were considered 
excessive for the job. The cyl- 
inders were also excessively 
heavy for convenient handling. 
Assuming that other researchers 
must have similar needs, the fol- 
lowing objectives were estab- 
lished for building a cylinder 
in the laboratory: (1) light 
weight, (2) low cost, (3) the ca- 



Ross A. Phillips 

pacity to produce the desired 
forces through the required dis- 
tance, and (4) a reasonable life 
expectancy but which could be 
a much shorter use period than 
normally specified for commer- 
cial cylinders. 

Cylinders for Agricultural 
Applications 

Most modern tractors have 
built-in hydraulic systems. These 
operate a special cylinder or 
cylinders integral with the trac- 
tor. Many systems have con- 
nections to operate a remote cyl- 
inder. The cylinders integral 
with the tractors have adequate 
production volume and use to 
justify special designs and 
honed finishes. The remote cyl- 
inders frequently have limited 
use on any one machine, due to 
seasonal operations, and are 
built in two standard sizes so 
that one cylinder may be used 
in numerous places. This justi- 
fies a more expensive cylinder 
and reduces costs due to the 
greater volume of production. 



The two sizes have 8 inch and 
16 inch strokes with extended 
lengths of 28.5 inches and 47.5 
inches and such diameters that 
the rate of operation is within 
specified limits. Thus, a remote 
cylinder for a tractor with a hy- 
draulic pump having a given 
volumetric displacement would 
not be interchangeable with a 
tractor having a hydraulic pump 
with a considerably different 
volumetric displacement. Most 
machines that need hydraulic 
controls can be designed to 
comply with these standard cyl- 
inders. Low cost special cyl- 
inders would find additional 
uses where the standard cyl- 
inders will not fit into a design. 
Also, these cylinders could 
open a new area for farm me- 
chanization in which a tractor 
is not involved. Equipment in- 
tegral with a structure is one 
area where this would apply. 

Industrial Applications 

Hydraulic cylinders of almost 
any size and shape can be found 
in various industrial uses. These 
have honed working surfaces. 
Frequently the working surfaces 
have noncorrosive and hardened 
platings which contribute to the 
long life of the cylinder. Special 
molded packings are used in 
conjunction with these excellent 
finishes with the result of 
trouble-free performance for 
millions of cycles. These fine 
finishes are very expensive, but 
quite desirable where continued 
use makes them economically 



feasible. Such extended lasting 
qualities are not necessary with 
most agricultural equipment. A 
specific installation may have 
less than 15,000 cycles in a 10- 
year period and be used as much 
as four times daily. Thus, a 
well made cylinder in a similar 
installation would become obso- 
lete long before it would wear 
out. 

Efficiency of Speed Force Ratios 

High-pressure pumps operate 
at favorable efficiencies which 
may be as high as 95 per cent. 
Cylinders also have good ef- 
ficiencies. Linear motion at any 
relatively low velocity may be 
obtained in one step from the 
pump to the cylinder. This re- 
quires the proper diameter of 
cylinder for a given pump ca- 
pacity. By similar reasoning, 
any desired force may be ob- 
tained with a given amount of 
power. 

Major power losses occur in 
the fluid flow between the pump 
and cylinder. These may be con- 
trolled and held within desired 
limits by use of adequate lines. 
The relative efficiency of a 
mechanical and a hydraulic sys- 
tem to provide a high mechani- 
cal advantage is much in favor 
of the hydraulic system. 

Specialized and 
Experimental Use 

The design of this cylinder 
was such as to provide for gen- 
eral application in a variety of 
uses. Facilities available in 




u 



Hi 
O 



different shops may require al- 
teration of the design to some 
extent. Machine work was held 
to a minimum. Materials used 
are readily available in most 
locations and in small quanti- 
ties. The objectives of (1) light 
weight, (2) low cost, (3) desired 
capacity, and (4) reasonable 
short life were all incorporated 
into the design. The final re- 
sult is illustrated in Figure 1. 
Dimensions can be varied, with- 
in structural limits, to any de- 
sired size. 

Lightweight was accomplish- 
ed by using aluminum. Extruded 
tubing and extruded solid stock 
was supplied by the Aluminum 
Company of America. The tubing 
alloy was 6062-T6. The solid 
stock alloy was 2017-T4. Mate- 
rial cost for this would be high 
per pound, but reduced machine 
work would cancel the material 
cost. The total estimated cost 
of materials was $40. Each alu- 
minum alloy had a yield point of 
40,000 psi. A working stress of 
10,000 psi was used for design; 
however, for the specific appli- 
cation of the cylinder being built, 
this never became critical. 

Cylinder Body 

Extruded finishes of the tube 
were used to reduce machine 
work. The inside tolerances 
were within + .001 as determined 
with a micrometer. A wall thick- 
ness of 0.250 inch was used to 
provide adequate rigidity. This 
cylinder wall would withstand a 
hydraulic working pressure of 



Extruded Finish — v 2+-I6N- 




FIGURE 2. Cylinder Body. 



2,500 psi, and the threaded ends 
were designed for this pressure. 
Threading and facing the ends 
was the extent of machine work 
on the body (Figure 2). 
Cylinder End 

The cylinder end away from 
the plunger was machined from 
solid aluminum stock (Figure 3). 
The "O" ring seal was placed 
so that no lateral pressure was 
on the machined part of the cyl- 
inder body. Some shops would 
prefer welded construction at 
this end, but threaded assembly 
had the advantage of providing 
for the removal of both ends for 
inspection of the cylinder body. 
Close tolerances were required 
only for the "0" ring groove. 

I Drill— \ +NPT 



Packing Gland 

Solid stock was used for this 
piece. Tubing with 1.5 inches 
inside diameter would have been 
as satisfactory as the solid 
stock. External threads and the 
"0" ring groove of this piece 
were identical with those of the 
cylinder end. The oil passage 
along the plunger rod was de- 
signed to have a cross section 
of the same area as available in 
one- quarter inch pipe fittings. 
Space was provided for one- 
quarter by one- quarter inch 
linear packing around the cyl- 
inder rod and clearance for the 
packing nut to compress this 
packing in the packing gland 
(Figure 4). 



2i-l6N l Drill - 




FIGURE 3. Cylinder End. 
4 




FIGURE 4. Packing Gland. 



Packing Nut 

Solid stock was again used, 
although tubing with 1.5 inches 
inside diameter would have been 
as satisfactory. Tolerance be- 
tween the plunger rod and the 
packing nut was held close so 
that the nut would act as a wiper 
to keep foreign matter away 
from the packing. The close fit 
also reduced the tendency for 
the packing to extrude (Figure 5). 



2g-l6N- 




FIGURE 5. Packing Nut. 



-i-IONC 



Plunger Rod 

Machine work was held to a 
minimum by using cold rolled 
steel and machining only the 
ends of the rod (Figure 6). A 
diameter of 1.5 inches provided 
a favorable 1/r ratio of less than 
170, assuming the cylinder body 
to be completely supported. A 
1.25-inch diameter would give a 
ratio of more than 200 when the 
plunger was extended. American 
Institute of Steel Construction 
working stresses allowed a load 
of 9,150 pounds with the plunger 
rod fully extended and with no 
eccentric loading. Since the 
maximum design load of the cyl- 
inder was 7,850 pounds, this 
would allow for a small eccen- 
tric load. Such loading could 
possibly be caused by friction 

Cold rolled finish 




FIGURE 6. Plunger Rod. 
5 



in the pins of a flexible linkage, 
or by tolerances of fabrication. 
This design load was approxi- 
mately one-fourth of the ultimate 
load as calculated by Euler's 
column formula. 

When the cylinder body is not 
supported and the rodend is con- 
sidered, the maximum column 
length increases from 63.5 inches, 
used for the above calculations, 
to 142.75 inches. The ultimate 
load calculated by Euler's col- 
umn formula, P =flx 2 El 
where \% 

P = load 
ft = factor for end conditions = 

1 for round ends 
E = modulus of elasticity , 

I = least moment of inertia in inches 4 
1 = length of column (inches) 

would be 3,490 pounds, and inde- 
pendent of the fiber stress of 
the steel rod. The modulus of 
elasticity used for steel was 29 
million pounds per square inch. 
An aluminum alloy (Alcoa 2017- 
T4) with a modulus of elasticity 
of 10.5 million pounds per square 
inch was also used forthe plung- 
er rod and had an ultimate 
strength of 1,265 pounds with 
the cylinder body unsupported. 
Obviously, these loads present 
great limitations for the cylinder 
under these particular conditions. 
However, for a long column the 
load increases as the inverse 
square of the length. As the 
cylinder contracts, the center of 
the column moves from near the 
end of the cylinder body where 
the plunger rod diameter con- 
trols the slenderness ratio, to- 



wards the center of the cylinder 
body. Thus, the ultimate load of 
the plunger rod would be approx- 
imately equal to the design load 
of the cylinder at an extension 
of 46 inches. Certain applica- 
tions could possibly be practical 
with the cylinder body unsup- 
ported if the actual load were 
always a reasonable amount be- 
low the ultimate load for any 
amount of extension. 

Plunger Assembly 

The plunger (Figure 7) was 
made relatively long to conform 
with standard practice. This 
helps to give stability when the 




FIGURE 7. Plunger. 

cylinder is extended. Tolerances 
for the outside diameter were 
held close to reduce tendencies 
for extrusion of the cups. The 
"0" ring was to seal the high 
pressure side from the low pres- 
sure side, and the location or 
type of this seal was not critical. 
The cup hub (Figure 8) on the 




FIGURE 8. Cup Hub. 



6 




FIGURE 9. Plunger Rod End. 

extension side of the plunger 
acted as a stop at full extension 
of the cylinder and gave protec- 
tion to the cup. The plunger rod 
supplied adequate protection for 
the other cup if the cylinder was 
contracted without the rod end 




FIGURE 10. Cup Hub. 
(Figure 9) which normally acted 
as a stop. The hubs (Figures 8 
and 10) were sized to hold the 
cups firmly without excessive 
force and to allow for slight 
lateral flexibility. Leather 
water pump cups were used as 
they were readily available and 
the assumption was made that 
leather would produce as much 
erosion on the cylinder wall for 
test purposes as any cup mate- 
rial. After some use, a cup ex- 
pander was added to the cup on 
the nut side of the plunger. The 
assembly was held together by 
a standard th re e-qu arter inch 
course thread hex nut. 



Cylinder Performance 

The cylinder was mounted on 
the experimental hay drier for 
which it was designed (Figure 
11). An automatic reversing 
valve, which is shown on the 
cylinder, kept the plunger cycling 
as long as fluid was supplied. 
A counter was connected to the 
reversing mechanism to obtain 
the number of cycles the cylin- 
der made. The pressure relief 
valve was never set above 1,000 
psi since this gave the maximum 
force desired on the hay drier. 

The cylinder was supported 
by the connecting pins as a 
short cylinder would normally 
be supported. When the plunger 
was extended, the 1,000 psi 
gave a force (3,140 pounds) near 
the ultimate and the cylinder 
would deflect considerably when 
eccentricity was applied by a 
lateral hand push on the cylin- 
der. No failure was encountered. 
Supports in a later model of the 
drier consisted of the pin at one 
end of the cylinder and a clamp 
to hold the rod end of the cylin- 
der in place. The pushing plate 
was equipped with loose fitting 
guides. This gave considerably 
more safety from any side thrust 
that would accidentally be ap- 
plied to the cylinder. 

An aluminum rod was tried 
with this mounting, but exces- 
sive deflection occurred before 
the desired loads were reached. 
This deflection was at loads be- 
low those calculated for long 
column action, which indicated 



that eccentricity was introduced 
by the pin connections. Controls 
were manually operated and 
guides afforded enough support 
that loads were removed before 
exceeding the elastic limit of 
the plunger rod. 

Expansion of the cylinder 
due to pressure was calculated 
to be one-thousandth of an inch 
at 5,000 psi. No measurement 
of change was obtained up to 
the 1,000 psi at which the cylin- 
der was operated. Thermal ex- 
pansion for aluminum is twice 
that of steel or an increase of 
one-thousandth of an i n c h on 
the cylinder diameter for approx- 
imately every 40° F temperature 
rise. The average temperature 
rise encountered with one horse- 
power input was slightly less 
than 30 ° F and the micrometer 
measurements indicated one- 



thousandth inch expansion. Most 
packing materials would accom- 
modate this thermal change of 
dimensions as the aluminum 
plunger would expand at approx- 
imately the same rate as the cyl- , 
inder. 

Total operation of the cylin- 
der has been an estimated 20,000 
cycles with varying pressures 
up to 1,000 psi. The counter was 
used only for some 300 cycles 
because the cylinder was re- 
quired to operate through a def- 
inite stroke to actuate the coun- 
ter. This hampered the practical 
use of the cylinder control valve. 
After one year's operation and 
approximately 2,000 cycles, the 
plunger cup used for extension 
was deformed so the lip of the 
cup did not seal on the cylinder 
wall. This was corrected with a 
cup expander. Measurements 




FIGURE 11. Hydraulic System for Pushing Hay Bales into an Experimental Drier. 



were made periodically with a 
micrometer on the inside diam- 
eter of the cylinder and on the 
diameter of the cold rolled steel 
plunger rod. Neither di ameter 
changed enough in the estimated 
20,000 cycles to indicate a 
change of dimensions. The 
wear was less than five ten-thou- 
sandths of an inch. After a few 
cycles (less than 100) of opera- 
tion, wear marks were clearly 
visible on both the plunger rod 
and on the inside of the cylinder 
indicating a seating of the 
plunger cups and the plunger rod 
packing. 

The packing nut required fre- 
quent adjustment to prevent oil 
leakage around the plunger rod. 
The first adjustment after reas- 
sembly was required between 50 
and 100 cycles and subsequent 
adjustments at approximately 
500-cycle intervals. A graph- 
ite-filled asbestos packing was 
used. Other types might require 
much less attention. The main- 
tenance presented no difficulty 
in the research operations. Re- 
placement of either packing or 
cups did not appear necessary 
before at least 100,000 cycles 
of operation. 

One precaution was used in 



storing the cylinder. The cyl- 
inder was contracted to prevent 
corrosion of the cylinder rod. 
The adhering oil would not give 
adequate protection and rusting 
occurred after an exposed period 
of one month in an unheated 
building. The length of permis- 
sible exposure would vary con- 
siderably with weather condi- 
tions. 

Summary 

The aluminum hydraulic cyl- 
inder with a cold rolled steel 
plunger rod was satisfactory for 
the application for which it was 
designed. The extruded finish 
of the tubing and the cold rolled 
surface of the steel gave ade- 
quate performance with the 
plunger cups and packing used. 

Factors which should be ob- 
served in the design of similar 
cylinders are (1) the low mod- 
ulus of elasticity of aluminum 
where column action becomes 
critical, and (2) the high thermal 
coefficient of expansion where 
excessive temperature changes 
are involved and a close fit is 
required with steel. Seals must 
accommodate the dimensional 
changes of either pressure or 
temperature.